This invention relates to abrasive grain that has improved suitability in the production of coated abrasives.
Coated abrasives are frequently made by depositing on a substrate a layer of a curable binder resin formulation referred to as a "maker" coat and then subsequently depositing on the uncured or incompletely cured maker coat, abrasive grains. The grains stick to the maker coat which is then either partially or completely cured before being provided with another coat of curable resin binder, (the "size" coat), overlaying the abrasive grain. This size coat, when cured in designed to hold the abrasive grain in place when the coated abrasive is in use.
This invention relates primarily to the technique for depositing the abrasive grain on the maker coat. There are two basic approaches: the grain can either be gravity fed on to the surface of the maker coat; or it can be projected on to the same surface. The latter technique is most widely used and the projecting force is electrostatic in nature. The grain is placed on a bed or tray and the substrate with the maker coat thereon is passed over the bed with the maker coat facing downwards. An electrostatic differential is established between plates behind the substrate, (and directly over the bed), and under the bed itself. The abrasive particles become charged and are projected towards the maker coat and adhere thereto. This is often referred to as UP-deposition.
That is the theory and for many abrasive grains it is quite effective. However some abrasive grains, such as particularly alumina/zirconia abrasive grains, are very susceptible to the moisture content of the air in terms of the amount of charge they can hold and the time for which the charge can be held. It is known to treat the abrasive grains with a salt such as magnesium chloride to improve the projectability but the moisture effect is a dominant factor even then. Often if the humidity of the projection area is allowed to vary the amount of grain deposited in any operation will be different at the end from what it was at the beginning or the grain coverage will be patchy. The standard solution to this problem is to use only freshly treated grain and/or to deposit the grain within a temperature and humidity controlled environment.
The seriousness of the problem is in part dependent on the size of the grain. Clearly the larger the grain, the more charge must be retained to enable it to be successfully projected. The problem seems to reach its most serious expression with alumina/zirconia grains of 24 grit size and coarser.
The present invention provides an abrasive grain that maintains a relatively constant projectability in a range of humidity conditions. The invention also provides a method of treating abrasive grain to improve its projectability. While the invention has broad applicability, it is primarily useful with grains that are moisture susceptible such as alumina/zirconia grains.